Syrian government forces tanks drive in the village of Tal Jabin, north of the embattled city of Aleppo.
Photograph: George Ourfalian/AFP/Getty Images

Opposition forces in northern Syria say they are losing their grip on Aleppo as Russian bombardment and advances by pro-Assad militias come close to cutting their supply lines and besieging the city.

After a week of the most intensive bombardment of the five-year war, forces loyal to the Syrian leader are in control of most of the countryside immediately to the north.

Russian jets have pounded the area throughout the past week, as Syrian factions have gathered in Geneva for a faltering peace summit. The Russian defence ministry said on Thursday it had hit almost 900 targets in Syria in the previous three days.

It also accused Turkey of preparing for a military incursion. “The Russian defence ministry registers a growing number of signs of hidden preparation of the Turkish armed forces for active actions on the territory of Syria,” said spokesman Igor Konashenkov.

The Russian air attacks have succeeded in clearing rebel strongholds that had defied two earlier regime pushes, and allowed loyalist forces led by Lebanese Hezbollah and Shia militias to advance towards a large industrial area at the gateway to the rebel-held east that has been transformed into a wasteland over three years of bombardment.

The fall of Aleppo would be a devastating blow to anti-Assad forces. Opposition groups, among them the al-Qaida aligned Jabhat al-Nusra, which sent large numbers of fighters to the city last week, have controlled Aleppo’s eastern half since the summer of 2012. Syrian forces, heavily backed by their allies have remained in control of the west.

“They have not stopped bombing,” said one rebel leader, who was in the process of leaving his position in the town of Hreitan. “All the hospitals have been destroyed. We have around seven attacks an hour every day for a week. There were more than 120 on Tuesday alone.”

Opposition groups said thousands of residents of the Aleppo countryside were headed on foot for the Turkish border, a journey of up to 50 miles. The Turkish prime minister, Ahmet Davutoğlu, estimated that as many as 70,000 newly displaced people were trying to reach the main border crossing at Killis.

Roads to the south of the adjoining Syrian town of Azaz were attacked by Russian jets earlier this month. “They have done all they can to destroy supply lines,” said one resident. “The world has fast forgotten that we were the ones who kicked out Isis two years ago. We have kept them out of the area since then.”

The stepped-up Russian attacks come despite Moscow’s stated commitment to a political process to end the war in Syria, which has been responsible for the greatest humanitarian crisis of modern times and laid large parts of the country to waste.

Russia’s intervention last October had the declared goal of battling the Islamic State terror group, which controls much of eastern Syria and has pockets of influence in the centre of the country and near Damascus.

However, military observers in the region and the US claim that at least 70% of airstrikes have instead targeted opposition groups fighting to oust the Syrian leader. Airstrikes have been particularly intense around Hama in the west, the southern town of Deraa and Sheik Miskeen and in the north.

Responding to the regime advances on Aleppo, the US secretary of state, John Kerry, said: “The continued assault by Syrian regime forces – enabled by Russian airstrikes – against opposition-held areas, as well as regime and allied militias’ continued besiegement of hundreds of thousands of civilians, have clearly signalled the intention to seek a military solution rather than enable a political one.

“We call upon the regime and its supporters to halt their bombardment of opposition-held areas, especially in Aleppo, and to lift their besiegement of civilians in accordance with UN security council resolutions.”

Officials in Ankara and Riyadh, which back the Syrian opposition, say Russia’s widespread support of Assad has ensured that his forces, which were reeling prior to Moscow’s intervention, are no longer at risk of losing the war.

“They are transparently winning the war for them by fighting the people who threaten Assad most,” said one Saudi official. “And that is not the terror group, Isis.”

Russian attacks in the south 10 days ago prompted another wave of refugees to leave for the Jordanian border. Aid agencies in Amman say the numbers of Syrians fleeing the war has increased markedly since Russian jets joined the fray.

Russian strikes have allowed regime forces to recapture Sheikh Miskeen near the Jordanian border. At the time, the UK special representative for Syria, Gareth Bayley, said: “Regime and Russian onslaught on the moderate opposition and civilians must stop. By continuing to support the regime in its bombardment of the moderate opposition, Russia risks damaging the already fragile process of intra-Syrian negotiations.

“I urge Russia to act on its stated commitment to achieving a political solution in Syria and to stop targeting the moderate opposition and killing civilians.”